Polarization retaining single-mode optical waveguide

ABSTRACT

A single mode optical waveguide is constructed in a manner such that the core thereof is subjected to a stress-induced birefringence. The fiber comprises an oblong core surrounded by an oblong inner cladding layer. An outer layer of stress cladding glass, which has a circular outer surface, surrounds the inner cladding layer. The TCE of the stress cladding glass is different from that of the inner cladding glass.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application Ser. No. 249,022entitled "Method of Making Polarization Retaining Single-Mode OpticalWaveguide," filed in the name of M. G. Blankenship et al. on even dateherewith, now U.S. Pat. No. 4,360,371.

BACKGROUND OF THE INVENTION

In many applications of single mode optical waveguides, e.g. gyroscopes,sensors and the like, it is important that the propagating opticalsignal retain the polarization characteristics of the input light in thepresence of external depolarizing perturbations. This requires thewaveguide to have an azimuthal asymmetry of the refractive indexprofile.

A slight improvement in the polarization performance of single modeoptical waveguides is achieved by distorting the fiber core symmetry asa means of decoupling the differently polarized waves. Two such opticalfiber waveguides are disclosed in U.S. Pat. No. 4,184,859 and in thepublication by V. Ramaswamy et al., "Influence of Noncircular Core onthe Polarisation Performance of Single Mode Fibers," ElectronicsLetters, Vol. 14, No. 5, pp. 143-144, 1978. However, the Ramaswamypublication reports that measurements on borosilicate fibers withnoncircular cores indicate that the noncircular geometry and theassociated stress-induced birefringence alone are not sufficient tomaintain polarization in single mode fibers.

The invention disclosed in U.K. patent application GB No. 2,012,983 A isbased upon the recognition that orthogonally polarized waves are moreefficiently decoupled in a waveguide that is fabricated in such a manneras to deliberately enhance stress-induced, or strain birefringence. Thatpatent teaches that such behavior is accomplished by introducing ageometrical and material asymmetry in the preform from which the opticalfiber is drawn. The strain-induced birefringence is introduced by atleast partially surrounding the single mode waveguide by an outer jackethaving a different thermal coefficient of expansion (TCE) than that ofthe waveguide and a thickness along one direction that is different fromits thickness along a direction orthogonal to the one direction. Forexample, the preform may be a three-layered structure comprising aninner core region surrounded by a cladding layer which is in turnsurrounded by an outer jacket layer having a TCE different than that ofthe cladding layer. Diametrically opposed portions of the outer layerare ground away, and the resultant preform is drawn into a fiberapproximating a slab configuration in which the thicknesses of the outerjacket layer are different in two orthogonal directions. A similarresult can be accomplished by constructing the preform from an innercore region, a cladding region and two outer jacket layers oppositelydisposed along the longitudinal surface of the preform. Difficulty canbe encountered in the manufacture of that type of preform since stressis built up in the outer layer. When grinding the outer layer or whencutting slots therein, the built-up stress has a tendency to cause thepreform to break. Assuming that a fiber can be drawn from the preform,the stress-forming outer layer is far removed from the fiber core, andtherefore, the effect of the stress on the core is minimal.

In one embodiment of GB No. 2,012,983 A represented by FIGS. 10-15, arelatively thick substrate tube forms the outer portion of the opticalfiber. In order to impart to the fiber the desired characteristics,either the inner or outer surface of the substrate tube is non-circular.Because at least a portion of the substrate wall must be relativelythick, the efficiency of deposition is adversely affected. Also, sincethe substrate tube forms the outer, compressive layer of the fiber,commercially available tubes may not be usable in the process unlessthey fortuitously possess the desired expansion and/or viscositycharacteristics of the resultant fiber outer layer.

In a fiber such as that illustrated in FIG. 12 of GB No. 2,012,983 A,the outer layer 60 of cladding is referred to herein as the stresscladding. It has been found that the stress σ at the core of acircularly symmetric single mode optical waveguide fiber is equal to theproduct of f×g where f is a function of geometrical factors and g is afunction of glass factors. The function f is given by the equation##EQU1## where A_(sc) is the cross-sectional area of the stress claddingand A_(f) is the total cross-sectional area of the fiber. The function fcan therefore have a value such that 0<f<1. The function g is given bythe equation ##EQU2## where E is the effective elastic modulus of thefiber, Δ∝ is the difference between the TCE of the stress cladding andthe TCE of the remainder of the fiber, ΔT is the difference between thelowest set point of the glasses of which the fiber is comprised and roomtemperature and ν is Poissons ratio. Since the aforementioned definitionof stress σ generally applies also to non-symmetrical fibers such asthose disclosed in GB No. 2,012,983 A, it is necessary to maximize f toobtain the greatest core stress and thus obtain the greatest stressbirefringence. Values of f greater than 0.9 should be achieved toprovide maximum values of stress birefringence. The need to maximizefunction f is recognized in GB No. 2,012,983 A as evidenced by equations(7) and (8) thereof.

Another art-recognized design criteria for single mode opticalwaveguides is concerned with minimizing loss. A common method of formingsingle mode optical waveguide preforms is illustrated in FIG. 11 of GBNo. 2,012,983 A which shows a plurality of vapor deposited layers on theinner surface of a substrate tube. The purity of the substrate tube isgenerally not as high as that of the vapor deposited glass. Therefore,the vapor deposited core glass is isolated from the substrate tube by alayer of vapor deposited optical cladding glass of sufficient thickness.For a single mode fiber having a core cross-section which is circular ornearly circular, the radius r_(s) of the optical cladding should be atleast five times the radius r_(a) of the core. This estimate is based onthe findings reported in the publication: Electronics Letters, Vol. 13,No. 15, pp. 443-445 (1977). For fibers having cores of oblongcross-section, this relationship lacks meaningful significance. In sucha fiber, the extent of the optical cladding is better described in termsof its thickness. Since the size of a single mode core is related to thetransmission wavelength λ, the thickness of the optical cladding canalso be specified in terms of λ. The aforementioned cladding radius tocore radius ratio implies that the thickness of the optical cladding beat least about 20λ. When a single mode waveguide is designed inaccordance with this criteria, loss associated with cladding thicknessis limited to an acceptably low value.

The following analysis of GB No. 2,012,983 A is made by taking intoconsideration, inter alia, the specific embodiment described inconjunction with FIGS. 10-12 thereof. The fiber of that embodiment willsatisfy the requirement that the ratio A_(sc) /A_(f) exceed 0.9 exceptwhen the substrate tube is completely filled with internal layers duringthe process of making the preform from which the fiber is drawn. Thisaforementioned exception is, of course, an impossibility. Since thesubstrate tube cannot be completely filled during the internal layerdeposition process, the total thickness of the internal layers islimited by the internal diameter of the substrate tube. It is well knownthat the core diameter of a step profile single mode fiber is about 3 to10 μm. The outside diameter of the fiber is typically about 125 μm. Ifthe preform described in GB No. 2,012,983 A is formed in accordance withconventional practice so that the ratio A_(sc) /A_(f) exceeds 0.9, thethickness of the optical cladding layer will be less than 20 λ atconventional wavelengths. Thus, the excess fiber loss due toinsufficient optical cladding thickness will not be sufficiently low formany applications.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved single polarization single mode optical waveguide exhibitingstress-induced birefringence. Another object is to provide a method ofmaking a polarization maintaining single mode optical waveguide whichcan be formed by a technique which does not weaken the preform.

The present invention relates to an optical waveguide comprising atransparent glass core having an oblong cross-sectional configuration.Disposed on the surface of the core is an eliptically-shaped layer ofoptical cladding glass having a refractive index greater than that ofthe core glass. Surrounding the eliptically-shaped layer of claddingglass there is disposed an outer layer of stress cladding glass having atemperature coefficient of expansion different from that of theeliptically-shaped cladding layer. The outer surface of the outercladding layer is substantially circular in cross-section.

The optical waveguide of the present invention is formed by providing atubular intermediate product comprising an inner layer of core glasssurrounded by a first cladding glass layer. The intermediate product iscollapsed to form a flattened preform foreproduct wherein the core glasshas been transformed into a unitary layer having an elongatedcross-section. This core layer is surrounded by an inner cladding layerwhich now has an oblong cross-sectional configuration. A layer of flamehydrolysis-produced soot is deposited on the outer surface of the innercladding layer, the TCE of the soot being different from that of theinner cladding glass. The resultant article is heated to consolidate thesoot into an outer cladding glass layer, thereby forming a solid glassdraw blank which can be drawn into an optical waveguide fiber.

In accordance with one method of forming the tubular intermediateproduct, a plurality of layers are deposited by a chemical vapordeposition technique on the inner surface of a substrate tube which isformed of a glass of lower purity than the glass layers depositedtherein. The innermost layer forms the core and the next adjacent layer,which is thicker than the core layer, forms the optical cladding. Thismethod of forming the fiber permits the thickness of the opticalcladding to be greater than 20λ at the operating wavelength. The core isthus adequately isolated from the impure substrate tube.

In another embodiment, the tubular intermediate product is formed by aflame oxidation technique. Reactant vapors are fed to a burner wherethey are oxidized in a flame to form glass soot which is deposited on acylindrical mandrel. The first applied soot layer forms the corematerial of the resultant fiber. At least one additional layer of sootis applied to the first layer to form the inner cladding. After themandrel is removed, the resultant hollow soot preform can beconsolidated to form a hollow glass tube which is thereafter heated onopposite sides to cause it to collapse flat. Alternatively, a lowpressure can be applied to the aperture of the soot preform to cause itto collapse flat during consolidation.

Both of these methods permit the formation of a very thick stresscladding layer so that the ratio A_(sc) /A_(f) is greater than 0.9.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of an intermediate product which isemployed in the formation of the preform from which the fiber of thepresent invention is formed.

FIG. 2 shows an apparatus for collapsing the intermediate product ofFIG. 1.

FIGS. 3 and 4 are schematic representations of an apparatus for forminga composite preform having an outer soot coating.

FIG. 5 is a cross-sectional view of a draw blank formed by consolidatingthe composite preform of FIG. 4.

FIG. 6 is a cross-sectional view of a single-mode single polarizationfiber drawn from the draw blank illustrated in FIG. 5.

FIG. 7 illustrates a flame hydrolysis process for forming a preformincluding a core portion and an inner cladding portion.

FIG. 8 shows the soot preform of FIG. 7 after the mandrel has beenremoved.

FIG. 9 shows the consolidated preform.

FIG. 10 is a schematic representation of a consolidation furnace whichmay be used to consolidate the preform of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The intermediate product 10 illustrated in FIG. 1 is formed by awell-known embodiment of the chemical vapor deposition technique wherebyone or more layers of glass are formed on the inside surface of asubstrate tube which later forms at least a portion of the claddingmaterial. The reactant vapor, together with an oxidizing medium, flowsthrough hollow, cylindrical substrate tube 12. The substrate and thecontained vapor mixture are heated by a source that moves relative tothe substrate in a longitudinal direction, whereby a moving hot zone isestablished within the substrate tube 12. A suspension of particulatematerial, which is produced within the hot zone, travels downstreamwhere at least a portion thereof comes to rest on the inner surface oftube 12 where it is fused to form a continuous glassy deposit. Suchprocess parameters as temperature, flow rates, reactants and the likeare discussed in U.S. Pat. No. 4,217,027.

A thin barrier layer 14 of pure silica or silica doped with an oxidesuch as B₂ O₃ is sometimes initially deposited on the inner surface oftube 12 which is usually formed of silica or a high silica contentglass, the purity of which is lower than that of the vapor-depositlayers formed therein. The barrier layer prevents the migration ofhydroxyl ions or other light absorbing impurities from tube 12 intooptical cladding layer 16. In order to reduce light transmission losscaused by the impurity of the substrate tube to an acceptably low level,the thickness of layer 16 is made sufficiently great that the thicknessof the optical cladding layer in the resultant fiber is greater than20λ. Since barrier layer 14 is optional, it is not shown in FIGS. 3-6.The optical cladding layer is a relatively thick layer of glass having arelatively low refractive index. It conventionally comprises pure silicaor silica doped with a small amount of a dopant oxide for the purpose oflowering processing temperatures. The addition of a small amount of P₂O₅ to the deposited silica cladding layer is taught in the publication:S. Sentsui et al., "Low Loss Monomode Fibers With P₂ O₅ -SiO₂ Claddingin the Wavelength Region 1.2-1.6 μm," 5th European Conference on OpticalCommunication, Amsterdam, September, 1979. The use of P₂ O₅ along witheither B₂ O₃ or F in the deposited silica cladding layer is taught inthe publication: B. J. Ainslie et al., "Preparation of Long Length ofUltra Low-Loss Single-Mode Fiber," Electronics Letters, July 5, 1979,Vol. 15, No. 14, pp. 411-413. The use of such dopants has resulted in adeposition temperature of about 1500° C., which is approximately 200° C.lower than the temperature required to deposit a pure fused silicacladding layer. Upon completion of optical cladding layer 16, arelatively thin layer 18 of core material is deposited on the innersurface thereof. Core layer 18 consists of a high purity glass having arefractive index greater than that of cladding layer 16. Layer 18conventionally comprises silica doped with a low-loss oxide for thepurpose of increasing the refractive index. Many dopants have beenemployed in the fabrication of the cores of single mode opticalwaveguide fibers, GeO₂ presently being preferred. Single mode waveguideshaving losses less than 1 dB/km in the infra red region comprise coresformed of SiO₂ doped with GeO₂ as reported in the aforementioned Sentsuiet al. and Ainslie et al. publications. The resultant intermediateproduct 10 contains an aperture 20.

For operation at wavelengths in the range between 1.1 and 1.8 μm apreferred intermediate product could be constructed in accordance withthe teachings of U.S. patent application Ser. No. 157,518 filed June 9,1980 in the name of P. E. Blaszyk et al. That application teaches theformation of a P₂ O₅ doped SiO₂ cladding layer on the inner surface of aborosilicate substrate tube followed by a thin layer of pure SiO₂ toprevent the P₂ O₅ from diffusing into the GeO₂ doped SiO₂ core which isdeposited on the inner surface of the pure SiO₂ layer.

For purposes of the present invention it is merely required thatintermediate product 10 comprise an inner layer of core glass surroundedby a layer of lower refractive index optical cladding glass. Core 18could be deposited directly on the inner surface of tube 12, forexample, if tube 12 were formed of high purity glass. As used herein,the term "inner cladding layer" means tube 12 and any other layer orlayers of glass surrounding core layer 18 in intermediate product 10.

It is an advantage of the present method that commercially availableglass tubes may be employed for substrate tube 12. The cross-sectionalarea of cladding layer 16 can be made much greater, e.g. more than twicethat of substrate tube 12 so that the physical characteristics ofdeposited layer 16, rather than those of tube 12, predominate in thedetermination of characteristics such as the thermal coefficient ofexpansion of the inner cladding. In such a situation, thecross-sectional area of the substrate tube is so small relative to thatof the entire resultant fiber that the physical characteristics thereofremain essentially insignificant.

Intermediate product 10 can be collapsed in the manner illustrated inFIG. 2. Burners 22 and 24 produce flames 26 and 28, respectively whichare directed onto opposite sides of intermediate product 10. During thisprocess intermediate product 10 may be mounted in the glass lathe (notshown) in which it was mounted during the formation of layers 14 and 16.During the collapse process illustrated in FIG. 2, lathe rotation ishalted so that only opposite sides of product 10 are heated. Thecollapsing step is preferably done under a controlled internal pressureas taught in U.S. Pat. No. 4,154,591. During this step, the heat sourcemust cover a sufficiently broad axial region of intermediate product 10to permit collapse thereof. Alternatively, a single heat source may beemployed in the manner described in U.S. Pat. No. 4,184,859, wherebyfirst one side and then the other is collapsed.

Complete collapse of the intermediate product 10 results in preformforeproduct 30 in which opposite sides of core layer 18 have beencombined to form a core portion 32 which is elongated in cross-section.Very large core aspect ratios can thus be achieved. The core issurrounded by inner cladding portion 34 and substrate portion 36, bothof which have an oblong geometry.

Preform foreproduct 30 is then provided with a cladding portion, theouter surface of which is substantially circular in cross-section. Thesurface of foreproduct 30 is prepared in a conventional manner prior todeposition of the outer cladding. The surface of preform foreproduct 30is kept clean after the firepolishing step which resulted in thecollapse of intermediate product 10 by inserting foreproduct 30 into aclean sealed bag such as a polyethylene bag. If foreproduct 30 ishandled or permitted to become dirty, several cleaning steps aretypically required. It is washed in deionized water and then washed inan isopropyl alcohol bath. It is then etched in HF to remove a fewmicrons of glass or about 1% of the article weight. Then foreproduct 30is rinsed in deionized water, degreased with isopropyl alcohol andplaced in a clean polyethylene bag. Soot of the desired glasscomposition is deposited on foreproduct 30 by a conventional flamehydrolysis process similar to that disclosed in U.S. Pat. Nos. 3,737,292and 4,165,223. Referring to FIGS. 3 and 4, there is shown an apparatuswhich is now conventionally employed in the manufacture of low-lossoptical waveguide fibers. A flame 38 containing glass soot emanates froma flame hydrolysis burner 40 to which fuel, reactant gas and oxygen orair are supplied. Burners such as those disclosed in U.S. Pat. Nos.3,565,345; 3,565,346; 3,609,829 and 3,698,936 may be employed. Liquidconstituents required to form the glass soot can be delivered to theburner by any one of the many well known reactant delivery systems knownin the prior art. Reference is made in this regard to teachings of U.S.Pat. Nos. 3,826,560; 4,148,621 and 4,173,305. Excess oxygen is suppliedto the burner so that the reactant vapors are oxidized within flame 38to form the glass soot which is directed toward foreproduct 30.

In accordance with one technique for forming the outer cladding layer,longitudinal strips 44 and 46 are initially deposited on the flattenedsidewalls of foreproduct 30 to accelerate the formation of a circularouter cladding. With the lathe halted, burner 40 makes a sufficientnumber of longitudinal passes to form a soot layer 44. Foreproduct 30 isrotated 180° and a second soot layer 46 is deposited opposite the firstone as shown in FIG. 4. Outer layer 48 of cladding soot is thendeposited by rotating foreproduct 30 while burner 40 traverses itlongitudinally.

The steps of depositing strips 44 and 46 of cladding glass may beomitted without affecting to too great an extent the geometry of theresultant fiber. If cladding layer 48 is deposited directly uponforeproduct 30, the soot stream from the burner will deposit a greateramount of soot when the flat side walls of foreproduct 30 are facing theburner than when the rounded portions thereof are facing the burnersince soot collection efficiency is a function of target size. Thistends to decrease the noncircularity of the soot blank cross-section aslayer 48 is built up. Substantial circularity should be achieved whenthe outside diameter of layer 48 is sufficient, relative to the size ofthe core, to enable the resultant fiber to function as a single-modefiber. The thickness of layer 48 must be sufficient to cause the ratioA_(sc) /A_(f) in the resultant fiber to exceed 0.9.

The flame hydrolysis-produced cladding layer is porous in form and mustbe heated to fuse or consolidate it into a glass layer free fromparticle boundaries. Consolidation is preferably accomplished bygradually inserting the composite body 50 into a consolidation furnacein the manner taught in U.S. Pat. No. 3,933,454. The resultant glassdraw blank 56 may not be circular if layers 44 and 46 are not applied orif they are applied in such a fashion that they do not balance theinitial non-circularity of preform foreproduct 30. The amount that theouter surface of consolidated blank 56 deviates from circularitydecreases with increasing amounts of outer cladding 48.

Draw blank 56 of FIG. 5 is inserted into a draw furnace wherein at leastone end thereof is heated to a temperature that is sufficiently high topermit fiber 70 of FIG. 6 to be drawn therefrom in accordance withconventional practice. During the drawing of fiber 70, surface tensiontends to round the outer surface thereof.

Alternative processes for forming an intermediate product areillustrated in FIGS. 7 through 10. As shown in FIG. 7, a first coating84 of glass soot is applied to cylindrical mandrel 85 by a conventionalflame hydrolysis process such as that referred to hereinabove. A flame86 containing glass soot emanates from flame hydrolysis burner 87 andimpinges upon mandrel 85. After a coating 84 of core glass is formed onmandrel 85, the composition of the reactant gas fed to burner 87 ischanged and a second coating 88 of inner cladding glass is applied tothe outer surface of first coating 84. The refractive index of coating84 is greater than that of coating 88. The physical characteristics ofcoating 88, such as the TCE thereof, are selected to impart the requiredamount of stress to the inner cladding of the resultant opticalwaveguide fiber.

After coating 88 has achieved the desired thickness, the mandrel isremoved as shown in FIG. 8 to form a porous preform 90 having anaperture 89. The resultant hollow soot preform can then be consolidatedin the manner described hereinabove to form hollow intermediate product10' as shown in FIG. 9. Intermediate product 10' can be collapsed in themanner illustrated in FIG. 2 and further processed in the mannerdescribed in conjunction with FIGS. 3 through 6 to form a polarizationretaining single-mode optical waveguide fiber.

The porous preform 90 illustrated in FIG. 8 can alternatively beconsolidated in the manner illustrated in FIG. 10 to form a preformforeproduct having a high aspect ratio core in a single processing step.After mandrel 85 has been removed from the soot preform, a tube 91 isinserted into one end of the preform. The preform is then suspended froma tubular support 92 by two platinum wires, of which only wire 93 isshown. The end of gas conducting tube 91 protrudes from tubular support92 and into the adjacent end of preform 90. The preform is consolidatedby gradually inserting it into consolidation furnace 94 in the directionof arrow 97. The preform should be subjected to gradient consolidation,whereby the bottom tip thereof begins to consolidate first, theconsolidation continuing up the preform until it reaches that endthereof adjacent to tubular support 92. During the consolidation processa flushing gas such as helium, oxygen, argon, neon, or the like, ormixtures thereof flow through the consolidation furnace as indicated byarrows 95. Prior to the time that preform 90 begins to consolidate,drying gases may be flowed into aperture 89 in the manner taught in U.S.Pat. No. 4,125,388. During the time that the initial tip of the preformbegins to consolidate, the pressure in aperture 89 is reduced relativeto that outside the preform. This may be accomplished by connecting avacuum system to gas conducting tube 91 by line 96. As preform 90 isinserted into the consolidation furnace in the direction of arrow 97 thelow pressure within aperture 89 causes aperture 89 to collapse flat,beginning in the region of the initially consolidated tip portion of thepreform. As the remainder of the preform becomes consolidated, theremainder of the aperture continues to collapse flat. Thus, in a singleconsolidation step, porous soot preform 90 having aperture 89 thereincan be consolidated and simultaneously have the aperture collapsed flatto form a preform foreproduct of the type illustrated by numeral 30 inFIG. 3.

Referring again to FIGS. 4-6, the composition of soot layer 48 (and thatof strips 44 and 46, if they are deposited) is such that the TCE of theresultant cladding layer 74 is much greater than or much less than theTCE of the remainder of fiber 70. It is known that portion 72(comprising core 80, substrate tube 82 and any layers forming innercladding 78) will be caused to be in tension if the TCE of the outer or"stress cladding" layer 74 is lower than the effective TCE of portion72. Conversely, portion 72 will be caused to be in compression if theeffective TCE thereof is lower than that of stress cladding layer 74.See the publication: S. T. Gulati and H. E. Hagy, American CeramicSociety 61 260 (1978). Moreover, a stress distribution will exist withinthe waveguide core 80 in which σ_(x) >σ_(y), where σ_(x) and σ_(y) arethe stresses in the core region parallel to and perpendicular to thelong axis of the core cross-section. Furthermore, this stress differencewill increase as the aspect ratio of the core region increases. Thisstress differential will produce the desired birefringence.

A stress of 20-40 kpsi in the core is needed to provide the requiredbirefringence. With the aspect ratios achievable by the processesdescribed hereinabove, the TCE difference between the inner cladding andthe outer stress cladding should be greater than 1×10⁻⁷ /°C. Followingare two theoretical examples wherein the glass compositions of thevarious parts of the fibers are chosen so that the fiber core is incompression and tension, respectively.

A fiber of the type shown in FIG. 6 is formed of the glass compositionsgiven in Table 1. The TCE of each composition is also listed.

                  TABLE 1                                                         ______________________________________                                        Composition (wt. %)                                                                    GeO.sub.2                                                                              SiO.sub.2                                                                            TCE (× 10.sup.-7 /°C.)                  ______________________________________                                        Core 80    15         85     13                                               Inner clad 78         100    5                                                Tube 82               100    5                                                Outer clad 74                                                                            30         70     23                                               ______________________________________                                    

The fiber defined by Table 1 has a core that is in compression and anouter cladding which is in tension. Although the core is adequatelystressed, this fiber may be undesirable from a strength standpoint. Sucha fiber could be strengthened by adding to the outer surface thereof afurther low expansion cladding layer of SiO₂, for example.

A fiber of the type illustrated in FIG. 6 could be formed of thematerials specified in Table 2 in order to put the core into a state oftension.

                  TABLE 2                                                         ______________________________________                                        Composition (wt. %)                                                                                               TCE                                              GeO.sub.2                                                                           P.sub.2 O.sub.5                                                                        SiO.sub.2                                                                            TiO.sub.2                                                                            (× 10.sup.-7 /°C.)           ______________________________________                                        Core 80  15      1.5      83.5        15                                      Inner clad 78    1.5      98.5        6                                       Outer clad 74             93   7      0                                       ______________________________________                                    

This type of fiber, in which the core is in tension, is preferred sincethe outer cladding will be in compression, a condition tending tostrengthen the fiber.

I claim:
 1. A polarization retaining single mode optical waveguide fibercomprisinga core of transparent glass, said core having an oblongcross-sectional configuration, an oblong inner cladding layer disposedon the surface of said core, said inner cladding including an opticalcladding layer of high purity glass surrounded by a layer of lowerpurity glass, the refractive index of said core glass being greater thanthat of said high purity and said lower purity cladding glasses, and anouter layer of stress cladding glass surrounding said inner cladding,said stress cladding glass having a thermal coefficient of expansiondifferent from that of said inner cladding glass, the outer surface ofsaid outer layer being substantially circular in cross-section.
 2. Afiber in accordance with claim 1 wherein the ratio of thecross-sectional area of the outer cladding layer to that of the entirefiber is greater than 0.9.
 3. A fiber in accordance with claim 2 whereinthe thickness of the optical cladding layer is greater than twenty timesthe wavelength at which the fiber is to be operated.
 4. A fiber inaccordance with claim 1 wherein the difference between the coefficientsof expansion of said stress cladding glass and said inner cladding glassis greater than 1×10⁻⁷ /°C.
 5. A fiber in accordance with claim 4wherein the thermal coefficient of expansion of said inner claddingglass is greater than that of said stress cladding glass.
 6. A fiber inaccordance with claim 1 wherein the optical purity of said outer layerof stress cladding glass is higher than that of the layer of low purityglass of said oblong inner cladding layer.
 7. A polarization retainingsingle mode optical waveguide fiber for use at at least one wavelengthλ, said fiber comprisinga core of transparent glass, said core having anoblong cross-sectional configuration, an oblong inner cladding layerdisposed on the surface of said core, said inner cladding including anoptical cladding layer of high purity glass, the refractive index ofwhich is lower than that of said core glass, the thickness of saidoptical cladding being greater than 20λ, and an outer layer of stresscladding glass surrounding said inner cladding layer, said stresscladding glass having a thermal coefficient of expansion different fromthat of said inner cladding glass, the outer surface of said outer layerbeing substantially circular in cross-section, the ratio of thecross-sectional area of the outer cladding layer to that of the entirefiber being greater than 0.9.
 8. A fiber in accordance with claim 7wherein the difference between the thermal coefficients of expansion ofsaid stress cladding glass and said inner cladding glass is greater than1×10⁻⁷ /°C.
 9. A fiber in accordance with claim 8 wherein the thermalcoefficient of expansion of said inner cladding glass is greater thanthat of said stress cladding glass.